Particulate Fuel Combustion Process and Furnace

ABSTRACT

The present invention provides a combustion furnace that comprises a combustion chamber defining a combustion zone within the combustion chamber and having at least one chamber wall facing the combustion zone, and at least one main particulate fuel burner mounted in a chamber wall and adapted to generate in the combustion zone a main flame directed away from the chamber wall by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein. With regard to the combustion chamber, the main particulate fuel burner has associated therewith at least one auxiliary burner mounted in the burner wall. The auxiliary burner is located in the vicinity of the main particulate fuel burner and is adapted to generate an auxiliary flame in the combustion zone located proximate the chamber wall and at least partially directed towards the main particulate fuel burner.

The present invention relates to the field of particulate fuelcombustion and the use of particulate fuel burners in industrialcombustion furnaces.

The present invention relates in particular to the use of particulatesolid fuel burners, such as particulate coal burners.

The present invention also relates to the use of particulate liquid fuelburners, whereby combustion is generated by injecting oxidant andparticulate liquid fuel, i.e. droplets of liquid fuel, into a combustionzone. The present invention relates more particularly to the use of suchparticulate liquid fuel burners for heavy liquid fuels.

Coal is the most abundant fossil fuel currently available. Most of thepower generated in the world uses coal as the fuel.

One way of generating heat or power is the use of coal burners.

In a coal burner, a conveying gas is often required to transport thesolid fuel particles from a fuel storage or milling device (e.g. a coalpulveriser) to the burner for subsequent combustion with an oxidant. Theoxidant for the combustion can be the conveying gas, a gas suppliedseparately from the conveying gas or a combination of the conveying gasand a separately supplied gas.

The combustion process of particulate solid fuel comprises severalcombustion steps which are described hereafter with reference to thecombustion of particulate coal:

-   -   1) The coal particles, which typically leave the storage or        milling device substantially at ambient temperature, are heated        to at least the devolatilization temperature of the coal. The        devolatilization temperature is the minimum temperature at which        devolatilization of the coal commences (see hereafter). The        devolatilization temperature may vary depending on the type or        grade (class) of coal, its humidity, etc.    -   2) When the temperature of the coal particles reaches the        devolatilization temperature of the coal, devolatilization of        the coal particles commences. Devolatilization is a process in        which the volatile components (in short: volatiles) of the coal        particles leave the coal particles in gaseous form. Volatiles        include highly combustible components such as hydrocarbons and        hydrogen.    -   3) The volatiles combust with the oxidant, thereby generating        heat and increasing the temperature of the remaining solid        matter or char.    -   4) Finally, the char combusts with remaining oxidant, thereby        generating further heat.

This multi-step combustion process distinguishes particulate solid fuelcombustion from the gaseous fuel combustion process in which the gaseousfuel combusts directly with the oxidant.

The particulate liquid fuel combustion process, in which liquid fuel isinjected into the combustion zone in the form of small particles ordroplets, is also a multi-step process. In a first step, the injectedliquid fuel droplets are heated to the evaporation temperature of thefuel when the fuel reaches its evaporation temperature, the liquid fuelevaporates to form inflammable fuel vapours and in the third step theinflammable fuel vapours combust with the oxidant and produce heat. Forlight fuels, such as domestic fuel oils, or No 1, 2 and 3 fuel oils, theevaporation temperature is relatively low and evaporation of the fuelinto vapours takes place almost instantly following injection into thecombustion zone at normal operational temperatures of most industrialfurnaces. Consequently, the combustion of particulate light liquid fuelsresembles that of gaseous fuels as far as rate of combustion followinginjection is concerned.

In the combustion of particulate medium heavy liquid fuels such as No 4fuel oil and very heavy liquid fuels such as residual fuel oil, or No 5and 6 fuel oils, the evaporation temperature is higher and evaporationof the liquid fuel takes place more slowly and more gradually. In thismanner, the combustion of particulate heavy and especially very heavyliquid fuels resembles the multi-step process of particulate solid fuelcombustion.

As a consequence, particulate solid fuel burners, such as particulatecoal burners, and particulate heavy liquid fuel burners, are usually notsuited for a narrow combustion chambers in which only short flames canbe used for heat generation.

Indeed, when the length of the flame exceeds the width of the combustionchamber (the width being the free dimension of the combustion chamberalong the flame axis), the flame impinges on the combustion chamberelement opposite the burner, thereby causing incomplete fuel combustionand fouling with partial-combustion products such as soot as well asthermal damage to the impinged chamber element. The said chamber elementcan be a chamber wall positioned opposite the burner, for example in aglass feeder or forehearth or in a reheat furnace. The element can alsobe a chamber element to be heated such as a radiant heating panel orpipes, such as boiler pipes, positioned opposite the burner within thecombustion chamber.

Air is traditionally used as the conveying gas and as the oxidant forparticulate fuel burners, as the conveying gas for solid particulatefuels and as the pulverisation gas for particulate liquid fuelinjectors. Burners using air as the oxidant for combustion are known asair-fuel burners.

In the case of oxy-fuel burners, the oxidant is an oxygen-rich gas (>25%vol O2) such as oxygen-enriched air or industrial oxygen having anoxygen content of at least 90% vol, preferably of at least 95% vol, andmore preferably of at least 98% vol.

The advantages of oxy-fuel burners over air-fuel burners are multiple:

-   -   improved fuel efficiency    -   improved heat transfer towards the charge to be heated,    -   reduced fumes generation,    -   higher CO2 concentration in the fumes, which is advantageous for        CO2 capture and sequestration    -   reduced pollutants emissions (e.g. NOx . . . ),    -   higher flame temperature providing improved the combustion of        hard-to-burn fuels,    -   etc.

In the case of oxy-fuel burners, the risk of thermal damage to theinstallation in case of narrow combustion chambers is particularlyimportant due to the higher flame temperature when compared to air-fuelburners.

Examples of narrow combustion chambers are side-fired tunnel or passagefurnaces, such as cement passage kilns, glass feeders or forehearths.

Other examples of narrow combustion chambers are side- and/orcross-fired vertical boilers and cracking installations (FCC).

In view of the high availability of solid fuels such as coal, includinglow-grade coal, and of heavy fuels, including combustible industrialliquid waste, often at advantageous prices, it would be highly desirableto be able to use particulate fuel burners in narrow industrialcombustion chambers.

This is made possible by the furnace according to the present inventionand the process of operating same.

The combustion furnace of the present invention comprises a combustionchamber defining a combustion zone therewithin and having at least onechamber wall facing the combustion zone.

At least one particulate fuel burner, hereafter referred to as “mainparticulate fuel burner”, is mounted in a chamber wall of saidcombustion chamber. Said main particulate fuel burner is adapted togenerate in the combustion zone a flame, referred to as “main flame”,directed away from said chamber wall by injecting oxidant in gaseousform and fuel in particulate form into the combustion zone forcombustion therein.

According to the invention, said main particulate fuel burner hasassociated therewith at least one auxiliary burner equally mounted insaid burner wall; Said auxiliary burner is furthermore located in thevicinity of the main particulate fuel burner with which it isassociated. The auxiliary burner is adapted to generate a flame,referred to as “auxiliary flame”, in the combustion zone located, saidauxiliary flame being proximate said chamber wall and at least partiallydirected towards said main particulate fuel burner.

Said main particulate fuel burner may have at least two auxiliaryburners associated therewith, for example, the main particulate fuelburner may be associated with two auxiliary burners, one on either side,or may be surrounded by four auxiliary burners.

As will be explained hereafter, an auxiliary burner may also beassociated with more than one main particulate fuel burner. For example,an auxiliary burner may be associated with two main particulate fuelburners, one on either side of the auxiliary burner.

The nominal power of the main particulate fuel burner is advantageouslygreater than the nominal power of the auxiliary burner. Preferably thenominal power of the main particulate fuel burner is at least 50%greater than the nominal power of the auxiliary burner and morepreferably at least twice the nominal power of the auxiliary burner.

It is also advantageous for the main particulate fuel burner and theauxiliary burner to be designed so that the momentum of the main flamegenerated by the main burner is higher than the momentum of theauxiliary flame generated by the auxiliary burner. Preferably, said mainflame momentum is at least 50% greater than auxiliary flame momentum,and more preferably at least twice the auxiliary flame momentum.

The main particulate fuel burner is preferably adapted to generate amain flame, the axis of which is situated in a vertical planeperpendicular to the chamber wall in which the main particulate fuelburner is mounted. For many applications it is advantageous for saidmain flame axis itself to be perpendicular to said chamber wall.

The fuel in particulate form injected by the main particulate burner canbe a particulate solid fuel or a particulate liquid fuel.

Examples of suitable particulate solid fuels are particulate coal, petcoke, combustible particulate solid waste. Different classes ofparticulate coal may be used depending on the process: lignite,bituminous coal or anthracite, from highly coking to non-coking coals,etc.

Particular examples of suitable liquid fuels are medium heavy liquidfuels, such as No 3 fuel oil, and heavy liquid fuels such as No 5 and No6 fuel oils, furnace fuel oils (FFO). The present invention isparticularly useful for the combustion of waste fuel or fuel waste, ifappropriate for the process concerned, in particular with regard to anyeffects on the charge to be heated.

The main particulate fuel burner may be an oxy-fuel burner. Inparticular, the main particulate fuel burner may be an oxy-fuel burneroperating with an oxidant, such as for example oxygen-enriched air,containing at least 50% by volume of oxygen, preferably at least 80% byvolume, more preferable at least 90% by volume of oxygen, or industrialoxygen having an oxygen content of at least 95% by volume, andpreferably of at least 98% by volume.

The auxiliary burner may or may not be a particulate fuel burner asdescribed above. The auxiliary burner may be connected to the same fuelsource as the main particulate fuel burner or to a different fuelsource, for example a gaseous fuel source.

The auxiliary fuel burner may be connected to the same oxidant source asthe main particulate fuel burner or to a different oxidant source.

The auxiliary fuel burner is advantageously an oxy-fuel burner.Preferably, both the main particulate fuel burner and the auxiliaryburner are oxy-fuel burners.

The furnace typically comprises a multitude of main particulate fuelburners, each main particulate fuel burner having at least one auxiliaryburner associated therewith. The furnace may thus comprise a multitudeof main particulate fuel burners in one chamber wall.

In some cases, for example certain vertical boiler furnaces, thecombustion furnace only has burners on one side of the combustionchamber.

In other cases, the combustion furnace has burners mounted in differentchamber walls.

The invention thus also relates to a combustion furnace, having at leastone main particulate fuel burner in a first chamber wall and at leastone particulate fuel burner in a second chamber wall positioned oppositethe first chamber wall across the combustion zone. Typically, such acombustion furnace will have a first multitude of main particulate fuelburners in the first chamber wall and a second multitude of mainparticulate fuel burners in the second chamber wall. Examples of suchcombustion furnaces include glass-melting furnaces, glass feeders orforehearths, tunnel calcination furnaces or reheat furnaces.

When several main particulate fuel burners are mounted in one wall,these burners are usually arranged in a geometric pattern.

For example, a furnace with a long narrow combustion chamber may have asubstantially horizontal row of main particulate fuel burners mounted inone or both lateral chamber walls. A furnace with a vertical narrowcombustion chamber may, for example, have main particulate fuel burnersarranged in a checkerboard pattern in a chamber wall.

The present invention also relates to a process of operating acombustion furnace. Said furnace comprises a combustion chamber defininga combustion zone within the combustion chamber. The combustion chamberhas at least one chamber wall facing the combustion zone. At least onemain particulate fuel burner is mounted in a chamber wall of thecombustion chamber. Said main particulate fuel burner is adapted togenerate in the combustion zone a main flame directed away from thechamber wall into which said main burner is mounted by injecting oxidantin gaseous form and fuel in particulate form into the combustion zonefor combustion therein. The main particulate fuel burner is associatedwith at least one auxiliary burner mounted in said chamber wall.

In the process of the invention, oxidant in gaseous form and fuel inparticulate form are injected into the combustion zone for combustiontherein by means of the main particulate fuel burner so as to generate amain flame in the combustion zone. Said main flame is directed away fromsaid chamber wall and has a flame root adjacent said main particulatefuel burner. Furthermore, according to said process, an auxiliary flameis generated in the combustion zone by means of the at least oneauxiliary burner associated with the main particulate fuel burner. Incontrast to the main flame, the auxiliary flame is located proximatesaid chamber wall and at is at least partially directed towards the rootof the main flame.

In this manner, the auxiliary flame provides additional heating for theparticulate fuel injected by the main particulate fuel burner. As aconsequence, the residence time required for said fuel particles toreach their devolatilization, respectively their evaporisationtemperature after their injection into the combustion zone is reduced,thus shortening the distance to be travelled by the fuel particles inthe combustion zone before combustion of the volatiles, respectively ofthe fuel vapours commences, which in term leads to a shortening of themain flame.

As already mentioned with respect to the furnace of the invention, themain particulate fuel burner may be associated with at least twoauxiliary burners.

According to one advantages embodiment, the auxiliary flame is a highlydivergent flame. For example, the auxiliary flame may have a flame axissubstantially perpendicular to the chamber wall a flame opening of atleast 90°, preferably of at least 110° and more preferably of at least130°. The flame opening is the cone angle α of the (truncated) conedefined by the root of the flame as shown in FIG. 1. Such an auxiliaryflame is very useful, when the auxiliary burner is associated withseveral main particulate fuel burners, as is the case according to oneembodiment of the invention. Indeed, when several main particulate fuelburners are positioned near to and around such an auxiliary fuel burner,the highly divergent flame is at least partially directed to the rootsof the main flames generated by each of said main particulate fuelburners.

According to an alternative embodiment of the process, the auxiliaryflame has a flame axis forming a sharp angle with said chamber wall. Theangle between said flame axis and the chamber wall is not more than 60°and more preferably not more than 30°. This is particularly suited forthe embodiment of the invention, according to which more than oneauxiliary burner is associated with one main particulate fuel burner.Indeed, several such auxiliary burners can be positioned around and inthe vicinity of one main particulate fuel burner, whereby each saidauxiliary burner generates an auxiliary flame directed at the root ofthe main flame generated by said main particulate fuel burner.

The main flame typically has a flame axis situated in a vertical planeperpendicular to said chamber wall. In many cases, it is advantageousfor the flame axis of the main flame to be substantially perpendicularto said chamber wall.

The purpose of the auxiliary burners is to accelerate the heating of theparticulate fuel by increasing the temperature in the vicinity of theroot of the main flame.

The auxiliary burner is usually operated continuously, as is the mainparticulate fuel burner.

In some cases, it is not necessary for the auxiliary burners to operateconstantly for the required shorter main flame length to be obtained. Inthat case, the auxiliary burner may be operated to generate theauxiliary flame intermittently.

The transfer of energy to the charge to be heated in the furnace ismainly effected by the one or more main flames, which are directed awayfrom the chamber wall towards the charge. It is consequentlyadvantageous for the main particulate fuel burner to operate at a powerwhich is greater than the power at which the auxiliary burner operates.Preferably the power of the main particulate fuel burner is at least 50%greater than the power of the auxiliary burner. More preferably thepower of the main particulate fuel burner is at least twice the power ofthe auxiliary burner.

In order to limit any deflection or disturbance of the main flame by theauxiliary flame, it is also advantageous for the momentum of the mainflame generated by the main burner to be higher than the momentum of theauxiliary flame generated by the auxiliary burner. Preferably, said mainflame momentum is at least 50% greater than the auxiliary flamemomentum, and more preferably at least twice the auxiliary flamemomentum.

As described above with respect to the furnace of the invention, thefuel injected by the main particulate fuel burner can be a particulatesolid fuel. It can also be a particulate liquid fuel.

The main particulate fuel burner is advantageously an oxy-fuel burner.

The auxiliary burner may or may not be a particulate fuel burner asdescribed above. The auxiliary burner may generate the auxiliary flameby injecting the same fuel as the main particulate fuel burner or byinjecting a different fuel, for example a gaseous fuel source.

The auxiliary fuel burner may generate the auxiliary flame using thesame oxidant as the main particulate fuel burner or using a differentoxidant.

The auxiliary fuel burner is advantageously an oxy-fuel burner.Preferably, both the main particulate fuel burner and the auxiliaryburner are oxy-fuel burners.

The invention is particularly useful for heating a charge in a narrowfurnace.

The process of the invention can, in particular be used to heat a chargein a furnace which only has burners on one side of the combustionchamber, as is the case in certain vertical boiler furnaces.

The process of the invention is also useful for heating a charge in afurnace having burners mounted in different chamber walls, for examplein combustion furnaces having at least one main particulate fuel burnerin a first chamber wall and at least one particulate fuel burner in asecond chamber wall positioned opposite the first chamber wall acrossthe combustion zone.

For many applications, the first and second walls are advantageouslylateral walls of the combustion chamber.

The process of the invention can in particular advantageously be used toheat a charge in glass-melting furnaces, in glass feeders orforehearths, in tunnel calcination furnaces and in reheat furnaces.

The main particulate fuel burner preferably comprises a burner blockdefining a passage therethrough for the injection of fuel and/or oxidanttherethrough into the combustion zone for generating the main flame. Theburner block, which is mounted in the chamber wall, is typically madeout of refractory material, such as AZS.

The auxiliary burner may likewise comprise a burner block defining aninjection passage therethrough.

According to a specific embodiment of the furnace and the process of thepresent invention, the auxiliary burner is mounted in the same burnerblock as the main particulate fuel burner with which it is associated.

The present invention and its advantages are illustrated in more detailhereafter, reference being made to the non-limiting FIGS. 1 to 3 and tothe examples, whereby:

FIG. 1 is a schematic cross section view of a chamber wall comprising arow of main particulate fuel burners and associated auxiliary burners,each main particulate fuel burner having two auxiliary burnersassociated therewith, and vice-versa,

FIG. 2 is a schematic view of the flame arrangement shown in FIG. 1along plane II-II as seen from the combustion zone,

FIG. 3 is a schematic view of an alternative flame arrangement as seenfrom the combustion zone, whereby one main particulate fuel burner hasfour associated auxiliary fuel burners.

In state-of-the-art combustion furnaces, the flame generated by saidburners is generally directed away from the furnace wall towards thecharge (for example, in the case of a vertical boiler furnace) orsubstantially parallel to the charge towards the opposite chamber wall(for example in a melting, fining or reheat furnace). In thesestate-of-the-art furnaces, a limited amount of heat is generated in thevicinity of the chamber walls. Indeed, high temperatures close to thechamber walls are generally avoided in order to prevent thermal damageto the refractory walls of the combustion chamber.

In the case of particulate fuel burners, and more specifically in thecase of solid particulate fuel burners and particulate fuel burners forheavier liquid fuel fractions, this method of operating a furnaceresults in long flames and/or incomplete fuel combustion due to the longresidence time/travel distance of the particulate fuel from its point ofinjection until it reaches its devolatilization/evaporation temperature.This makes these state-of-the-art furnaces badly suited for certainparticulate solid and liquid fuels, in particular when the furnace is anarrow one.

The present invention now makes it possible to obtain relatively shortparticulate fuel flames by reducing the residence time required for thefuel particles to reach their devolatilization, respectively theirevaporisation temperature after their injection into the combustionzone, thus shortening the distance travelled by the fuel particles inthe combustion zone before combustion of the volatiles, respectively ofthe fuel vapours commences, which in term leads to a shortening of theflame.

According to the present invention, this is achieved as follows.

The particulate fuel burner 100 of FIG. 1, has the burner block 110,made of the refractory material AZS and comprising a through passage 120into which are mounted, in concentric relationship, a central fuelinjector and a surrounding oxidant injector (not shown). Both injectorsand are made of the heat resistant alloy Inconel® 600

The central fuel injector may be a particulate solid fuel injectorthrough which particulate solid fuel is transported by means of aconveyor gas. The central fuel injector may also be a liquid fuelpulverizer, also known as “atomiser”, by means of which liquid fuel issprayed into the combustion zone in the form of particulate liquid fuel.The mechanism of injecting particulate liquid fuel used by the centralfuel injector may be mechanical spraying, gas assisted spraying or acombination of the two. Suitable known spray devices are described inEP-A-1750057 and WO-A-03006879.

The oxidant used contains 90% vol O₂.

Burner block 110 is mounted in chamber wall 10.

Main particular fuel burner 100 injects particulate fuel and oxidantthrough passage 120 into combustion zone 20 to generate main flame 130directed away from the chamber wall 10 towards the charge (for example,in the case of a vertical boiler furnace) or substantially parallel tothe charge towards the opposite chamber wall (for example in a melting,fining or reheat furnace). Main flame axis 131 is perpendicular tofurnace wall 10.

Auxiliary burner 200 is also a particulate fuel oxy-burner. Burner block210 is mounted in chamber wall 10 between two successive mainparticulate fuel burners 100.

The auxiliary flame 230 generated by auxiliary burner 200 is a very wideflame with low momentum. Auxiliary burner 200 furthermore operates atsubstantially less than the power of main particulate fuel burner 100.

As shown in FIGS. 1 and 2, auxiliary flame 230 is partially directed tothe root 135 of the main flame 130 generated by the two main particulatefuel burners 100 on either side of the auxiliary burner 200. In thismanner, auxiliary flame 230 accelerates the heating of the particulatefuel injected by main particulate fuel burner 100 as it leaves injectionpassage 120. As a consequence thereof, the injected particulate fuelreaches its devolatilization/evaporation temperature more rapidly thanwould have been the case without auxiliary fuel burner 200, the flamelength of the main flame is shortened and substantially complete fuelcombustion can be achieved in the main flame, even in narrow furnaces.Adequate flame shortening can even be achieved with intermittentoperation of auxiliary burner 200

Due to the low power and momentum of the auxiliary flame, this isachieved without significant disturbance to the main flame's form andflow pattern and without causing thermal damage to chamber wall 10 andmain burner block 110 due to overheating. Excellent overall furnaceefficiency can thus be obtained.

In the embodiment shown in FIG. 3, the main particulate fuel burnercomprising injection passage 120 is surrounded by four auxiliaryburners, each having an injection passage 320. Again, the main flame 130generated by the main particulate fuel burner is directed away from thechamber wall 10 towards the charge or substantially parallel to thecharge towards the opposite chamber wall. The auxiliary flame 230generated by the four auxiliary burners is a relatively narrow flame(small flame opening) having a flame axis 231 forming a sharp angle (notshown) with said chamber wall 10. and directed towards the root of mainflame 130. Again, the auxiliary flame 230 is a low-power, low-momentumflame compared to main flame 130.

Again, the flame length of the main flame is shortened and substantiallycomplete fuel combustion can be achieved in the main flame, even innarrow furnaces, and this with good overall furnace efficiency andwithout significant disturbance to the main flame's form and flowpattern and without thermal damage to the chamber wall.

1. A combustion furnace comprising: a combustion chamber defining acombustion zone within the combustion chamber and having at least onechamber wall facing the combustion zone, and at least one mainparticulate fuel burner mounted in a chamber wall and adapted togenerate in the combustion zone a main flame directed away from thechamber wall by injecting oxidant in gaseous form and fuel inparticulate form into the combustion zone for combustion therein,wherein the main particulate fuel burner has associated therewith atleast one auxiliary burner mounted in the burner wall, the auxiliaryburner being located in the vicinity of the main particulate fuel burnerand being adapted to generate an auxiliary flame in the combustion zonelocated proximate the chamber wall and at least partially directedtowards the main particulate fuel burner.
 2. The combustion furnace ofclaim 1, wherein the main particulate fuel burner has at least twoauxiliary burners associated therewith.
 3. The combustion furnace ofclaim 1, wherein the auxiliary burner is associated with more then onemain particulate fuel burner.
 4. The combustion furnace of claim 1,wherein the nominal power of the main particulate fuel burner is greaterthan the nominal power of the auxiliary burner.
 5. The combustionfurnace of claim 4, wherein the nominal power of the main particularfuel burner is at least 50% greater than the nominal power of theauxiliary burner.
 6. The combustion furnace of claim 4, wherein thenominal power of the main particulate fuel burner is at least twice thenominal power of the auxiliary burner.
 7. The combustion furnace ofclaim 1, wherein the main particulate fuel burner is a solid particulatefuel burner or a liquid particulate fuel burner.
 8. The combustionfurnace of claim 1, wherein the combustion furnace includes a multitudeof main particulate fuel burners, each main particulate fuel burnerhaving at least one auxiliary burner associated therewith.
 9. Thecombustion furnace of claim according to claim 8, having a multitude ofmain particulate fuel burners in one chamber wall.
 10. The combustionfurnace of claim 8, having at least one main particulate fuel burner ina first chamber wall and at least one particulate fuel burner in asecond chamber wall positioned opposite the first chamber wall acrossthe combustion zone.
 11. The combustion furnace of claim 9, having atleast one main particulate fuel burner in a first chamber wall and atleast one particulate fuel burner in a second chamber wall positionedopposite the first chamber wall across the combustion zone.
 12. Aprocess of operating a combustion furnace, wherein the furnacecomprises: a combustion chamber defining a combustion zone within thecombustion chamber and having at least one chamber wall facing thecombustion zone, and at least one main particulate fuel burner mountedin a chamber wall and adapted to generate in the combustion zone a mainflame directed away from the chamber wall into which the main burner ismounted by injecting oxidant in gaseous form and fuel in particulateform into the combustion zone for combustion therein, the processcomprising: injecting oxidant in gaseous form and fuel in particulateform into the combustion zone for combustion by means of the mainparticulate fuel burner so as to generate a main flame in the combustionzone directed away from the chamber wall, the main flame having a rootadjacent the main particulate fuel burner, wherein the main particulatefuel burner has associated therewith at least one auxiliary burnermounted in the chamber wall in the vicinity of the main particulate fuelburner, and the process further comprising the step of: generating anauxiliary flame in the combustion zone by means of the at least oneauxiliary burner associated with the main particulate fuel burner, theauxiliary flame being located proximate the chamber wall and at leastpartially directed towards the root of the main flame generated by themain particulate fuel burner.
 13. The process of claim 12, wherein themain particulate fuel burner has at least two auxiliary burnersassociated therewith.
 14. The process of claim 12, wherein the auxiliaryburner is associated with more than one main particulate fuel burner.15. The process of claim 13, wherein the auxiliary burner is associatedwith more than one main particulate fuel burner.
 16. The process ofclaim 12, wherein the auxiliary flame has a flame axis substantiallyperpendicular to the chamber wall a flame opening of at least 110°. 17.The process of claim 16, wherein the auxiliary flame has a flame axissubstantially perpendicular to the chamber wall a flame opening of atleast 130°.
 18. The process of claim 17, wherein the auxiliary flame hasa flame axis substantially perpendicular to the chamber wall a flameopening of at least 150°.
 19. The process of claim 12, wherein theauxiliary flame has a flame axis forming a sharp angle with the chamberwall.
 20. The process of claim 19, wherein the sharp angle with thechamber wall is not more than 60° with the chamber wall.
 21. The processof claim 20, the sharp angle with the chamber wall is not more than 30°with the chamber wall.
 22. The process of claim 12, wherein the fuelinjected by the main particulate fuel burner is a particulate solid fuelor the fuel burners is a particulate liquid fuel.
 23. The process ofclaim 12, wherein the main particulate fuel burners are oxy-fuelburners.